Seismic data acquisition may be conducted by towing some number of streamer sections behind a vessel. The streamer sections may have varying types of construction and sensor mounting in the streamer. Data recorded on these streamers are stored in memory on the towing vessel.
Marine seismic acquisition systems typically include, among other parts, controlled sources generating seismic waves (e.g., pulses) and receivers, also known as “hydrophones,” recording the waves reflected at each interface between sub-surface layers. These receivers are embedded in long cables called streamers which are towed in parallel by seismic vessels, together with other components, thereby making up a streamer spread. Ranging from 4 up to 12 kilometers, the streamers are separated from each other in the cross line direction by the use of deflectors located at each end of the spread generating a side force called lift.
It is desired to tow a seismic spread as quickly as safety and efficiency will allow. It is also desirable to tow the spread to cover as much lateral distance as safety and efficiency will allow. Some issues relating to safety are the maximum tension that the spread can withstand. As the spread is towed faster, drag increases. If tension at any portion of the spread becomes too large for the relevant component, such can break. Also, the distance between streamers (often towed in parallel, but also towed in other configurations such as “fan” configurations) relates to potential for entanglement. Since acts of nature and errors in calculation and control can make positioning of streamer imprecise and reduced at times, a shorter distance between the streamers can increase risk of entanglement. Similar issues are also faced by other components of the spread, including deflectors, tail buoys, etc.
While these issues have been addressed when towing a spread in a straight line, they have not been addressed when a vessel tows a spread along a nonlinear course such as a turn, or in a circular pattern (coil shooting). In those nonlinear towing cases, the outer portion of the spread (with respect to the arc of the turn) will experience an increased velocity through the water with respect to the interior portions of the spread. This increased velocity can result in increased drag and tension on components of the spread. As noted, the increased drag and tension can increase the potential for component failure. Also, the portions of the spread in the interior can move slowly and therefore become more susceptible to natural forces such as currents.
One way to mitigate the negative factors experienced by the increased velocity of the outer portions of the spread (fast moving) and the inner portions of the spread (slow moving) is by reducing the lateral outward distance of the spread, i.e., to narrow the spread. However, doing such introduces various issues that have not yet been addressed, such as efficient transition between linear towing into, through and out of a turn with respect to minimizing possibility of entanglement, maximizing speed and efficiency, and also reducing possibility of equipment failure.
Various embodiments herein address a number of these issues.